This invention relates to the field of intraocular surgery, and in particular to surgical instruments used to irrigate and simultaneously aspirate body fluids during intraocular surgery.
In eye surgery, the need often arises to remove fluids from the eye or to break up and remove tissue or material from the eye. For example, the need for such removal of material and fluids commonly arises during lensectomies wherein the lens of the eye is removed and replaced. Various techniques have been developed whereby needles or cannula can be inserted into various portions of eye in order to break up or remove these bodily tissues and fluids.
Several methods of breaking up and removing ocular materials require the insertion or addition of fluids to irrigate the surrounding portions of the eye. The fluid is then aspirated, usually simultaneously with the irrigation, to remove the materials which had been loosened and are entrained within the irrigation fluid. This simultaneous irrigation/aspiration operation can be achieved by using two coaxial cannula with the inner cannula providing the aspiration while the outer cannula provides the irrigation, or vice vera.
The inner and outer cannula essentially form a tube within a tube with a very small space in between the two tubes. This small space between tubes makes it difficult to surgically sterilize the instruments after they have been used.
The sterilization problems are minimized by using a disposable outer cannula which is removed and discarded after use so that the inner cannula can be sterilized and reused. The disposable outer cannula is often made of a flexible elastomeric material such as silicon. The thin silicon cannula can be integrally molded to a silicon base which slips over the tip of the surgical instrument supporting the inner cannula. The flexible silicone forms an effective leak proof seal to insure predictable fluid flow during irrigation.
A silicone cannula and support structure has the advantage of being relatively inexpensive, but more importantly, the cannula can bend freely. Surgeons sometimes bend the inner and outer cannulas to facilitate access to various portions of the eye, as when the surgeon must reach over the bridge of the nose to reach the medial portions of the eye. The inner cannula is usually thin enough to bend without buckling or crimping. A silicone outer cannula can freely bend without permanently buckling, although buckling and flow restriction do occur.
A flexible, outer silicone cannula also makes it easy to change the spacing between the irrigation and aspiration apertures when the cannulas are bent. The distance between the aspiration aperture and the irrigation apertures can be varied by screwing the inner cannula inward or outward. This screw extension and retraction requires the cannulas to rotate. When the inner cannula is bent, then in order to rotate, the inner cannula must bend or rotate the outer cannula. Since the outer cannula is fastened to the tip of the surgical instrument, it cannot rotate. A rubber outer cannula can be freely bent by the inner cannula to allow the rotational extension or retraction. A metal outer cannula is not bent so easily, and may be too stiff to be bent by the inner cannula. Repeated bending of the outer metal cannula also raises the risk of metal fatigue and fracture.
A flexible, outer silicone cannula also makes it easy to rebend and re-orient the cannulas. Some surgical irrigation/aspiration instruments allow the inner cannula to be rotated. Once the inner cannula is bent, rotating of the inner cannula to facilitate repositioning by the surgeon is hindered unless the outer cannula can freely rotate. Metal outer cannulas usually inhibit rotational adjustment when they are bent since the inner cannula is often not strong enough to bend the outer cannula as the inner cannula is rotated. Silicone outer cannulas do not inhibit this rotational adjustment since the rubber freely bends.
The silicone rubber outer cannula is not without its problems, however. The rubber cannot be molded as thin as metal. Thus rubber cannulas require larger incisions in order to be inserted into the eye. Also, the rubber is not as smooth as metal, which can create problems in obtaining a smooth, clog free fluid flow, and can possibly tear the soft tissue sliding along the length of the rubber cannula. Further, the rubber does not buckle during bending so as to restrict fluid flow. On occasion, the rubber outer cannula has buckled or axially collapsed during insertion into the eye with the result that the collapsed rubber tears the eye since it is too large and rought to fit smoothly through the incision in the eye. The buckling collapse is especially prone to occur at the tip of the outer cannula where the irrigation apertures are located.
Many surgeons also prefer a more rigid material than the soft silicon rubber since it is occasionally necessary to suture the sclera or other portions of the eye around the outer cannula with the result that a suture can radially compress the soft silicon cannula and cut off fluid flow through the outer cannula. Using a metal outer cannula overcomes the concern that the sutures may restrict the flow between the inner and outer cannula. Unfortunately, making the outer cannula out of surgically compatible metal is expensive. Additionally, the common way of fastening a metal outer cannula and its metal base to the surgical instrument is by threading the base portion of the outer cannula. This threading operation contributes to the increased cost of the outer cannula.
Using an outer cannula and support base made entirely of metal also presents problems when the inner and outer cannulas are bent by the surgeon in order to facilitate surgical access to various portions of the eye. Bending the cannula usually buckles and crimps the outer cannula and restricts the fluid flow, especially the fluid flow between the inner and outer cannula. Repeated bending of the inner and outer cannula can result in fatigue and fracture of the metal outer cannula. The outer cannula usually fatigues and fractures before the inner cannula since it is of larger diameter and thus is inherently less flexible than the thinner inner cannula.
There have been recent attempts to commercially produce a surgical instrument containing a metal inner and outer cannula wherein the entire surgical instrument is disposable. Making the entire instrument disposable is not only more expensive, but results in the waste of a significant amount of materials compared to only disposing the outer cannula and its support base.
There is thus a need for a lightweight, disposable outer cannula which has sufficient radial stiffness to allow suturing around the cannula without restricting the fluid flow, yet which can allow the outer cannula to be bent with minimum obstruction to the fluid flow between the inner and outer cannula. This cannula must be inexpensive to make yet still form an effective, leak proof seal when attached to the surgical instrument.